In a transmission of communication signals, especially analogue telephone signals and/or ISDN data signals, pairs of copper lines which each connect a subscriber to a master station location are used. Switching of the corresponding communication signals to a number of subscribers takes place in the master station location. For this purpose, so-called line cards are usually provided in the master station location; these cards supply a port for each of the subscribers.
In order to grant the individual subscribers access to a broadband communication network in addition to analogue telephone services or ISDN services, e.g. for Internet applications, it is usual additionally to apply broadband communication signals to the copper lines. In this so-called DSL transmission method (Digital Subscriber Line transmission method), a frequency range is used which is located above that used for analogue telephone signals and/or ISDN data signals. To achieve decoupling of the low frequency range communication signals for analogue telephone applications and ISDN applications, so-called splitters are used. The splitters are a suitable combination of high-pass and low-pass filters, and are always necessary in the master station location and usually at the subscribers too. Conventionally, a telephone/ISDN line card is provided in the master station location for a group of subscribers, as well as a DSL modem for each subscriber with access to the broadband communication network, this equipment being connected via a splitter to the respective port assigned to the subscriber. The provision of access to the broadband communication network by means of a DSL transmission method for a subscriber thus requires the installation of a splitter and a DSL modem in the master station location, and thus involves a considerable cost in both material and working time. In particular, this cost arises again for each new access to be provided. This in turn means that the provision of access to a broadband communication network for the corresponding subscriber typically involves comparatively high costs.
Known in this context from the EP 1 156 655 A2 is an interface system for use in a master station location, where the connection of a DSL modem to a telephone line card is developed in such a way that an external splitter can be dispensed with. This is achieved by the provision of a reactive impedance on the subscriber access side of a DSL coupling transformer, this impedance taking over the splitter function. This reduces the number of components to be installed in the master station location, thus especially reducing the time required for installation and maintenance, and also the material cost.
In the use of a splitterless interface system described above, there is however a problem in that for each new access to be supplied, i.e. for each subscriber, a DSL modem must also be supplied in the master station location, or, if the DSL modem is provided as an integral function of the telephone line card, the DSL modem is installed in advance for each subscriber. There is thus a problem in this case too in that for each access to the broadband communication network to be supplied, essentially the same costs arise.
One approach for solving the aforementioned problem when supplying accesses to the broadband communication network is to set up a point-to-multipoint connection which provides that a single DSL modem in the master station location serves several subscribers, as is the case, for example, in so-called broadcast DSL transmission methods. A common transmission path is partially used here for transmission of the communication signals. Since a DSL modem does not have to be supplied in the master station location for each subscriber in this case, the cost of supplying accesses to a broadband communication network can be further reduced.
The structure of a communication network with a DSL point-to-multipoint connection according to the prior art is shown by way of example in FIGS. 6a and 6b. Here a device for transmission of communication signals or line card, e.g. a telephone or ISDN line card 1′, is connected to a number of subscribers 2. In a transmission path, a splitter 4 is inserted between the subscribers 2 and the line card 1′ in each case. The splitters 4 each have a low-pass output, via which a connection to the line card 1′ is established, and a high-pass output for the connection to a DSL modem.
In the variants shown in FIG. 6a, the high-pass outputs of the splitters 4 are interconnected, achieving a signal summation in the DSL frequency band. The interconnected high-pass outputs are connected to a DSL modem 3′ developed especially for use with point-to-multipoint connections. A broadband communication signal which is transmitted from the DSL modem 3′ to one of the subscribers is therefore presented to all subscribers 2. In a transmission of the broadband communication signal in the reverse direction, i.e. from one of the subscribers 2 to the DSL modem, a split of the common transmission path is necessary. This can happen, for example, with an access control which allocates the subscribers 2 time slots for accessing the transmission path, e.g. a so-called TDMA access control (‘Time Domain Multiple Access’ access control), or can be achieved with a so-called CDMA access control (‘Code Division Multiple Access’ access control), in which signals of all subscribers 2 are simultaneously transmitted and the communication signals of each subscriber 2 are each individually given a corresponding CDMA code and a switch of carrier frequencies follows corresponding to the CDMA code.
FIG. 6b shows a technically more costly variant, in which the merging of the high-pass outputs of the splitters 4 occurs within the DSL modem 3″.
Since in the solutions presented above an external splitter has to be installed for each of the subscribers in the master station location, there exists a problem of a high material and time cost for the supply of the accesses to the broadband communication network.